The best place to sell items for gil are the Auction Houses found in FFXI. People sell their by Price History, and the cheapest items always go fast. It is usually a good idea to sell your item priced just under the Price History to ensure ffxi quick gil. Furthermore, if items are native to a starting town then it is much more profitable to sell them in other starting towns.
To earn good amounts of gil, hunt creatures which are most profitable. Such beasts include Giant Bees, Goblin Thugs, Crawlers, and Spiny Spipi. For earning while sleeping, you can even set macros to fish for you while you sleep. It is best to go for some Moat Carp as these sell best. If you need help, Skryer sells excellent guides to macros, fishing, and more.